The gas turbine community continually seeks to increase the thermal efficiency and power output by increasing the turbine inlet temperature to beyond the melting temperature of turbine airfoil vanes and blades. Effective cooling schemes are required to protect the gas turbine components from failure. Many cooling techniques for example film cooling, pin fins cooling and rib-turbulated cooling are employed to protect the airfoils, preventing the airfoils from failure while extending durability.
According to EP 1 508 746 A1 a heat exchange wall includes a base plate, a plurality of first protrusions distributed on a surface of the base plate, and a plurality of second protrusions distributed on the base plate surface. The height of the second protrusion in a normal direction of the base plate is desirably less than ½ of a height of the first protrusion in the normal direction. The height of the second protrusion in the normal direction is desirably between 1/20 and ¼ of the height of the first protrusion in the normal direction. More desirably, the height of the second protrusion in the normal direction is 1/10 of the height of the first protrusion in the normal direction.
According to the document ASME 2001-GT-0178 pin fins are normally used for cooling the trailing edge region of a turbine, where their aspect ratio (height H/diameter D) is characteristically low. In small turbine vanes and blades, however, pin fins may also be located in the middle region of the airfoil. In this case, the aspect ratio can be quite large, usually obtaining values greater than 4. Heat transfer tests, which are conducted under atmospheric conditions for the cooling design of turbine vanes and blades, may overestimate the heat transfer coefficient of the pin-finned flow channel for such long pin fins. The fin efficiency of a long pin fin is almost unity in a low heat transfer situation as it would be encountered under atmospheric conditions, but can be considerably lower under high heat transfer conditions and for pin fins made of low conductively material.
Referring to ASME GT 2011-46078 a pin-fin array is usually rows of short circular cylindrical elements generally arranged in staggered configurations in a narrow channel with cooling fluid passing over the array. This appears to be an effective heat transfer enhancement method, but is accompanied with a pressure loss. Pin fins are usually attached perpendicularly to both end-walls inside the narrow cooling channel, for example of a gas turbine airfoil. According to this document, FIG. 2 shows schematically a pin-rib geometry viewed from the top and the side of the channel. Various Figures show the top view of the top end-wall mounted with pin-fins. A further Figure illustrates the side view of the staggered pin-fins configuration in the test section. The top and bottom end-wall are identical and the bottom end-wall is arranged by shifting on pitch downstream of the top end-wall.
Generally, referring to the pressure loss coefficient it is noted that the heat transfer enhancement is usually accompanied by penalty of additional pressure loss. Any element protruding from the end-wall, i.e. pin fins and ribs, will obstruct the flow causing drag and head loss in the system.